Detecting endianness programmatically in a C program

Question

Is there a programmatic way to detect whether or not you are on a big-endian or little-endian architecture   I need to be able to write code that will execute on an Intel or PPC system and use exactly the same code  i e  no conditional compilation

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I surprised no-one has mentioned the macros which the pre-processor defines by default. While these will vary depending on your platform; they are much cleaner than having to write your own endian-check.

For example; if we look at the built-in macros which GCC defines (on an X86-64 machine):

:| gcc -dM -E -x c - |grep -i endian
#define __LITTLE_ENDIAN__ 1

On a PPC machine I get:

:| gcc -dM -E -x c - |grep -i endian
#define __BIG_ENDIAN__ 1
#define _BIG_ENDIAN 1

(The :| gcc -dM -E -x c - magic prints out all built-in macros).

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Declare   My initial post is incorrectly declared as  compile time   It s not  it s even impossible in current C   standard  The constexpr does NOT means the function always do compile-time computation  Thanks Richard Hodges for correction   compile time  non-macro  C  11 constexpr solution   union     uint16 t s    unsigned char c 2     constexpr static  d  1    constexpr bool is little endian       return d c 0     1

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untested  but in my mind  this should work  cause it ll be 0x01 on little endian  and 0x00 on big endian   bool runtimeIsLittleEndian void     volatile uint16 t i 1   return    uint8 t   amp i  0   0x01   0x01 little  0x00 big

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The C   way has been to use boost  where preprocessor checks and casts are compartmentalized away inside very thoroughly-tested libraries   The Predef Library  boost predef h  recognizes four different kinds of endianness   The Endian Library was planned to be submitted to the C   standard  and supports a wide variety of operations on endian-sensitive data   As stated in answers above  Endianness will be a part of c  20

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You can do it by setting an int and masking off bits  but probably the easiest way is just to use the built in network byte conversion ops  since network byte order is always big endian    if   htonl 47     47          Big endian   else        Little endian      Bit fiddling could be faster  but this way is simple  straightforward and pretty impossible to mess up

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int i 1  char  c  char   amp i  bool littleendian c

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I would do something like this   bool isBigEndian         static unsigned long x 1       static bool result reinterpret cast lt unsigned char  gt   amp x  0     0       return result      Along these lines  you would get a time efficient function that only does the calculation once

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You can use std  endian if you have access to C  20 compiler such as GCC 8  or Clang 7    Note  std  endian began in  lt type traits gt  but was moved to  lt bit gt  at 2019 Cologne meeting  GCC 8  Clang 7  8 and 9 have it in  lt type traits gt  while GCC 9  and Clang 10  have it in  lt bit gt     include  lt bit gt   if constexpr  std  endian  native    std  endian  big           Big endian system   else if constexpr  std  endian  native    std  endian  little           Little endian system   else          Something else

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Do not use a union   C   does not permit type punning via unions  Reading from a union field that was not the last field written to is undefined behaviour  Many compilers support doing so as an extensions  but the language makes no guarantee   See this answer for more details   https   stackoverflow com a 11996970    There are only two valid answers that are guaranteed to be portable   The first answer  if you have access to a system that supports C  20  is to use std  endian from the  lt type traits gt  header      At the time of writing  C  20 has not yet been released  but unless something happens to affect std  endian s inclusion  this shall be the preferred way to test the endianness at compile time from C  20 onwards    C  20 Onwards  constexpr bool is little endian    std  endian  native    std  endian  little       Prior to C  20  the only valid answer is to store an integer and then inspect its first byte through type punning  Unlike the use of unions  this is expressly allowed by C   s type system   It s also important to remember that for optimum portability static cast should be used  because reinterpret cast is implementation defined      If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined          a char or unsigned char type    C  11 Onwards  enum class endianness       little   0      big   1      inline endianness get system endianness         const int value   0x01        const void   address   static cast lt const void   gt   amp value       const unsigned char   least significant address   static cast lt const unsigned char   gt  address       return   least significant address    0x01    endianness  little   endianness  big      C  11 Onwards  without enum   inline bool is system little endian         const int value   0x01        const void   address   static cast lt const void   gt   amp value       const unsigned char   least significant address   static cast lt const unsigned char   gt  address       return   least significant address    0x01       C  98 C  03  inline bool is system little endian         const int value   0x01      const void   address   static cast lt const void   gt   amp value       const unsigned char   least significant address   static cast lt const unsigned char   gt  address       return   least significant address    0x01

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How about this    include  lt cstdio gt   int main         unsigned int n   1      char  p   0       p    char   amp n      if   p    1          std  printf  Little Endian n        else          if    p   sizeof int  - 1     1              std  printf  Big Endian n            else             std  printf  What the crap  n        return 0

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For further details  you may want to check out this codeproject article Basic concepts on Endianness      How to dynamically test for the Endian type at run time       As explained in Computer   Animation FAQ  you can use the   following function to see if your code   is running on a Little- or Big-Endian   system  Collapse   define BIG ENDIAN      0  define LITTLE ENDIAN   1    int TestByteOrder        short int word   0x0001     char  byte    char     amp word     return byte 0    LITTLE ENDIAN   BIG ENDIAN          This code assigns the value 0001h to a   16-bit integer  A char pointer is then   assigned to point at the first    least-significant  byte of the   integer value  If the first byte of   the integer is 0x01h  then the system   is Little-Endian  the 0x01h is in the   lowest  or least-significant    address   If it is 0x00h then the   system is Big-Endian

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union       int i      char c sizeof int      x  x i   1  if x c 0     1      printf  little-endian n    else    printf  big-endian n      This is another solution  Similar to Andrew Hare s solution

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while there is no quick and standard way to determine it  this will output it    include  lt stdio h gt   int main           unsigned int i   1      char  c    char   amp i      if   c              printf  Little endian        else        printf  Big endian        getchar        return 0

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I was going through the textbook Computer System  a programmer s perspective  and there is a problem to determine which endian is this by C program   I used the feature of the pointer to do that as following    include  lt stdio h gt   int main void       int i 1      unsigned char  ii    amp i       printf  This computer is  s endian  n     ii 0   1     little     big         return 0      As the int takes up 4 bytes  and char takes up only 1 bytes  We could use a char pointer to point to the int with value 1  Thus if the computer is little endian  the char that char pointer points to is with value 1  otherwise  its value should be 0

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Please see this article      Here is some code to determine what is   the type of your machine  int num   1  if   char    amp num    1        printf   nLittle-Endian n      else       printf  Big-Endian n

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See Endianness - C-Level Code illustration      assuming target architecture is 32-bit   4-Bytes enum ENDIANNESS  LITTLEENDIAN   BIGENDIAN   UNHANDLE      ENDIANNESS CheckArchEndianalityV1  void         int Endian   0x00000001     assuming target architecture is 32-bit             as Endian   0x00000001 so MSB  Most Significant Byte    0x00 and LSB  Least     Significant Byte    0x01        casting down to a single byte value LSB discarding higher bytes          return    char     amp Endian    0x01    LITTLEENDIAN   BIGENDIAN

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Unless the endian header is GCC-only  it provides macros you can use    include  endian h      if    BYTE ORDER      LITTLE ENDIAN          else if    BYTE ORDER      BIG ENDIAN          else   throw std  runtime error  Sorry  this version does not support PDP Endian

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bool isBigEndian         static const uint16 t m endianCheck 0x00ff       return      uint8 t   amp m endianCheck     0x0

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The way C compilers  at least everyone I know of  work the endianness has to be decided at compile time  Even for biendian processors  like ARM och MIPS  you have to choose endianness at compile time  Further more the endianness is defined in all common file formats for executables  such as ELF   Although it is possible to craft a binary blob of biandian code  for some ARM server exploit maybe   it probably has to be done in assembly

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As stated above  use union tricks   There are few problems with the ones advised above though  most notably that unaligned memory access is notoriously slow for most architectures  and some compilers won t even recognize such constant predicates at all  unless word aligned   Because mere endian test is boring  here goes  template  function which will flip the input output of arbitrary integer according to your spec  regardless of host architecture    include  lt stdint h gt    define BIG ENDIAN 1  define LITTLE ENDIAN 0  template  lt typename T gt  T endian T w  uint32 t endian           this gets optimized out into if  endian    host endian  return w      union   uint64 t quad  uint32 t islittle    t      t quad   1      if  t islittle   endian  return w      T r   0          decent compilers will unroll this  gcc         or even convert straight into single bswap  clang      for  int i   0  i  lt  sizeof r   i              r  lt  lt   8          r    w  amp  0xff          w  gt  gt   8            return r       Usage   To convert from given endian to host  use   host   endian source  endian of source   To convert from host endian to given endian  use   output   endian hostsource  endian you want to output   The resulting code is as fast as writing hand assembly on clang  on gcc it s tad slower  unrolled  amp   lt  lt       for every byte  but still decent

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Here s another C version  It defines a macro called wicked cast   for inline type punning via C99 union literals and the non-standard   typeof   operator    include  lt limits h gt    if UCHAR MAX    UINT MAX  error endianness irrelevant as sizeof int     1  endif   define wicked cast TYPE  VALUE           union     typeof   VALUE  src  TYPE dest       src   VALUE    dest    Bool is little endian void        return wicked cast unsigned char  1u       If integers are single-byte values  endianness makes no sense and a compile-time error will be generated

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Declare an int variable   int variable   0xFF    Now use char  pointers to various parts of it and check what is in those parts   char  startPart   reinterpret cast lt char  gt    amp variable    char  endPart   reinterpret cast lt char  gt    amp variable     sizeof  int   - 1    Depending on which one points to 0xFF byte now you can detect endianness  This requires sizeof  int     sizeof  char    but it s definitely true for the discussed platforms

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If you don t want conditional compilation you can just write endian independent code  Here is an example  taken from Rob Pike    Reading an integer stored in little-endian on disk  in an endian independent manner   i    data 0  lt  lt 0     data 1  lt  lt 8     data 2  lt  lt 16     data 3  lt  lt 24     The same code  trying to take into account the machine endianness   i      int  data    ifdef BIG ENDIAN    swap the bytes    i     i amp 0xFF  lt  lt 24       i gt  gt 8  amp 0xFF  lt  lt 16       i gt  gt 16  amp 0xFF  lt  lt 8       i gt  gt 24  amp 0xFF  lt  lt 0    endif

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As pointed out by Coriiander  most  if not all  of those codes here will be optimized away at compilation time  so the generated binaries won t check  endianness  at run time   It has been observed that a given executable shouldn t run in two different byte orders  but I have no idea if that is always the case  and it seems like a hack to me checking at compilation time  So I coded this function    include  lt stdint h gt   int   BE   0   int is big endian         if   BE    0            uint16 t  teste    uint16 t  malloc 4            teste     teste  amp  0x01FE    0x0100          uint8 t teste2     uint8 t   teste  0           free teste            BE    int  malloc sizeof int              BE    0x01    teste2             return   BE      MinGW wasn t able to optimize this code  even though it does optimize the other codes here away  I believe that is because I leave the  random  value that was alocated on the smaller byte memory as it was  at least 7 of its bits   so the compiler can t know what that random value is and it doesn t optimize the function away   I ve also coded the function so that the check is only performed once  and the return value is stored for next tests

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You can also do this via the preprocessor using something like boost header file which can be found boost endian

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Ehm    It surprises me that noone has realized that the compiler will simply optimize the test out  and will put a fixed result as return value  This renders all code examples above  effectively useless  The only thing that would be returned is the endianness at compile-time  And yes  I tested all of the above examples  Here s an example with MSVC 9 0  Visual Studio 2008    Pure C code  int32 DNA GetEndianness void        union                uint8  c 4           uint32 i        u       u i   0x01020304       if  0x04    u c 0           return DNA ENDIAN LITTLE      else if  0x01    u c 0           return DNA ENDIAN BIG      else         return DNA ENDIAN UNKNOWN      Disassembly  PUBLIC   DNA GetEndianness   Function compile flags   Ogtpy   File c  development dna source libraries dna endian c     COMDAT  DNA GetEndianness  TEXT   SEGMENT  DNA GetEndianness PROC                   COMDAT    11         union    12             13             uint8  c 4     14             uint32 i    15           u    16        17         u i   1    18        19         if  1    u c 0     20             return DNA ENDIAN LITTLE       mov eax  1    21         else if  1    u c 3     22             return DNA ENDIAN BIG    23         else   24            return DNA ENDIAN UNKNOWN    25            ret  DNA GetEndianness ENDP END   Perhaps it is possible to turn off ANY compile-time optimization for just this function  but I don t know  Otherwise it s maybe possible to hardcode it in assembly  although that s not portable  And even then even that might get optimized out  It makes me think I need some really crappy assembler  implement the same code for all existing CPUs instruction sets  and well     never mind   Also  someone here said that endianness does not change during run-time  WRONG  There are bi-endian machines out there  Their endianness can vary durng execution  ALSO  there s not only Little Endian and Big Endian  but also other endiannesses  what a word    I hate and love coding at the same time

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This is normally done at compile time  specially for performance reason  by using the header files available from the compiler or create your own  On linux you have the header file   usr include endian h

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I don t like the method based on type punning - it will often be warned against by compiler  That s exactly what unions are for    bool is big endian void        union           uint32 t i          char c 4         bint    0x01020304        return bint c 0     1       The principle is equivalent to the type case as suggested by others  but this is clearer - and according to C99  is guaranteed to be correct  gcc prefers this compared to the direct pointer cast   This is also much better than fixing the endianness at compile time - for OS which support multi-architecture  fat binary on Mac os x for example   this will work for both ppc i386  whereas it is very easy to mess things up otherwise

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Unless you re using a framework that has been ported to PPC and Intel processors  you will have to do conditional compiles  since PPC and Intel platforms have completely different hardware architectures  pipelines  busses  etc  This renders the assembly code completely different between the two   As for finding endianness  do the following   short temp   0x1234  char  tempChar    char   amp temp    You will either get tempChar to be 0x12 or 0x34  from which you will know the endianness

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